The present invention relates to telecommunications of digital signals. In particular, the present invention relates to coding a digital signal and decoding of the coded digital signal without error and loss of information.
Digital coding is applied widely for the transmission of signals over optical, cable, radio connections and other transmission media. Coding is applied to transmitted signals for several reasons. For example, coding helps retain the quality of the digitally coded signal after transmission. It can also hide the content of the coded message or signal. Coding can also protect the coded message against interference or jamming, and can increase the capacity of the transmission medium to allow the medium to handle a greater number of messages or signals.
More particularly, the present invention relates to the area of ternary and higher, multi-value digital scrambling methods and apparatus as well as descrambling methods and apparatus. The method and apparatus of the present invention which applies methods that generate digital sequences which are the coded form of digital message to be transmitted or which form a substantial basis for creating coded messages.
Typical digital transmission systems have transmitters and receivers. The original message to be transmitted can be any type of signal, such as voice, video, text or any other data format. The signal is digitized, coded and then modulated to be transmitted over the transmission medium. At the receiving end, the signal is demodulated, decoded and then applied to some device to re-constitute the original message format.
The digitized message has been generally represented in binary (or 2-value) format. This means that the signal provided to the coder is a binary signal and the signal provided to the decoder is also a binary signal.
There are different ways to implement binary coding and decoding. In the prior art, a well established way to code a binary message is by applying a binary scrambler. A binary scrambler is an electronic device or an executed method that has as its input a series or sequence of binary digits, which will be guided through a finite length shift register or a computer program that acts as such. The content of pre-established cells of the shift register will be tapped and connected to an element that will conduct a specific binary operation on two binary elements. In the prior art, this operation is overwhelmingly Modulo-2 addition. The result of the Modulo-2 addition can be fed back into the shift register, fed into a next Modulo-2 addition or being sent to the modulation stage of the transmission.
If the input to a binary scrambler is a sequence of binary digits, the output of a binary scrambler is also a series or sequence of binary digits. The input and output of a scrambler are in general and preferably dissimilar. The dissimilarity of the input and the output sequences depends on the input sequence, the length of the shift register, the number and place of taps and applied Modulo-2 add operations and initial content of the shift register.
The descrambler has as its input the output of the scrambler. The descrambler reverses the operation of the scrambler and can recover, without mistakes, the original uncoded digital message that formed the input to the scrambler.
The common element in all binary scramblers and descramblers are the binary Modulo-2 additions. The binary logical operation is also known under its binary logic designation: Exclusive OR or XOR or ≠.
Scramblers and descramblers are currently binary methods or devices that are composed of binary XOR functions. Binary XOR functions have the property of Modulo-2 addition. Modulo-2 addition is identical to Modulo-2 subtraction. In general, a binary signal is scrambled by adding it to another known binary signal under Modulo-2 rules. The original binary signal can be recovered from the scrambled signal by Modulo-2 subtraction of the known binary signal from the scrambled signal. Because Modulo-2 addition and Modulo-2 subtraction are both represented by the binary logic XOR function, binary scrambling and descrambling take place by the same binary logic function. While Modulo-2 addition is identical to Modulo-2 subtraction, that identity is not true for Modulo-3 and higher Modulo-n addition and subtraction. It is apparent to the inventor that the more fundamental description of identical scrambling/descrambling functions is that two binary inputs A and B generate a binary output C. If A and C are the inputs to the function, the output B is generated. Or if B and C are the inputs to the function, A is generated as the output. Binary scrambling functions can also be applied in binary Direct Sequence Spread Spectrum coding where an initial binary digital sequence is combined with a secondary, binary sequence with substantially more digits through a binary scrambling function.
In many cases it would be beneficial, either to the performance, quality or capacity of the telecommunication system to transmit modulated digital signals that represent higher value digits. For instance a cable system may transmit 3-value or ternary signals to balance the Direct Current component of the signals in the transmission system. These ternary signals can assume one of three states. The use of multi-value (greater than 2) signals also can increase the capacity (in information rate, or number of users) of a communication system. Nevertheless, many systems limit themselves to operating in a binary fashion. This is because of the availability and pervasiveness of binary technology and the lack of ternary and higher multi-value methods and technologies.
The availability of ternary (or 3-value) methods (even if executed in binary fashion) would greatly improve the performance of digital systems. Signal coding is an example of an area that would be greatly improved by the use of ternary or multi-value scrambling techniques. Also, as higher value scramblers have a greater number of scrambling functions, the application of ternary and higher value scramblers can make a spread-spectrum signal much harder to decode for un-authorized users and equipment. Application of ternary and higher-value scramblers can substantially improve the performance, capacity and security of digital communication systems.
In these respects and in others, the ternary and multi-value scrambling methods according to the present invention substantially depart from the conventional concepts, and provide a method primarily developed to conduct ternary and multi-value scrambling and descrambling of digital signals.